Method for producing molded parts for low-voltage, medium-voltage and high-voltage switchgear

ABSTRACT

The invention relates to a method for producing molded parts for low-voltage, medium-voltage and high-voltage switchgear and to the corresponding switchgear according to the preamble of patent claims  1, 2, 3  and  17 . The aim of the invention is to remove the disadvantages mentioned in the description while providing an advantageous method. According to the invention, a mixture of spheres having a defined distribution of diameters of a size Dx are introduced into the casting compound and the components are cast directly.

The invention relates to a method for production of insulating moldingsfor switching devices for low-voltage, medium-voltage and high-voltage,and to a switching device itself, as claimed in the precharacterizingclauses of patent claims 1, 2, 3 and 17.

The components of the switching devices mentioned are subject to verystringent requirements. In addition to the required dielectriccharacteristics, mechanical characteristics such as cantilever strength,impact resistance and the inclination to form cracks, etc, are ofimportance at the same time.

Previous experience relating to this has indicated the possibility ofcrack formation in epoxy resin components and components composed ofother insulating materials in switching devices such as these.

It is essential to avoid this. Efforts have already been made in thepast to achieve this. Vacuum chambers and other parts which wereinstalled in the dielectric moldings were encapsulated directly into theload-bearing enclosure composed of epoxy resin, together with theirfixed and moving connections. In order to prevent crack formation inthis case, the materials of the moldings are encapsulated jointly with afilling powder added to it composed of quartz powder or synthetic silicaflour.

This procedure has been proven.

Furthermore, components have been encapsulated in silicone orpolyurethane or a “soft” casting resin without any filling powder, inorder to increase the external dielectric strength.

The encapsulation technique means that the vacuum switching chamber andthe inserted parts must be cushioned, for mechanical reasons, by meansof an elastomer material before introduction into epoxy resin. Therequirements for this material are:

-   -   high dielectric strength    -   good adhesion to the vacuum switching chamber (or to the        inserted part)    -   good adhesion to the surrounding epoxy resin    -   adequate elasticity to absorb thermal stresses and mechanical        stresses.

The purpose of this cushioning is to absorb stresses that are created inthe component during production and operation of the epoxy resincomponents as a result of mechanical or thermal shrinkage.

The components have a correspondingly high total weight as a result ofthe high density of the filling powders or else filling powder mixturesthat are normally used nowadays.

When using silicone or polyurethane, or “soft” casting resin without anyfilling powder, the mechanical strength of the completely encapsulatedcomponents is small, and is of a resilient nature.

Against this background, the invention is based on the object ofimproving a method of this generic type such that the stateddisadvantages are overcome, while at the same time retaining thedescribed advantages obtained.

The stated object is achieved for a method of this generic type by thecharacterizing features of patent claim 1.

Other advantageous refinements are specified in the dependent claims 4to 16.

With regard to a switching device of this generic type, the statedobject is achieved by the characterizing features of patent claim 17.

Further advantageous refinements of the device according to theinvention are specified in the other dependent claims.

The essence of the invention relating to the method is in this case thata mixture of balls with a statistical distribution of diameters of sizeDx are introduced as a filler into the encapsulation compound. The useof balls, glass balls or hollow glass balls as a filler in epoxy resinor in plastics, or else a combination of balls and filling powders,allows the chemically dependent shrinkage during curing to be set to beconsiderably less than the values currently achieved in the references,and it is even possible to reduce the coefficient of expansion of thefinished component. Higher packing densities are achieved with the useaccording to the invention of a mixture with statistical particleexternal diameters. The particles or particle matrix are or is thusdenser, or are or is distributed more densely. This results inmechanically more-resistance direct encapsulation or directencapsulation of parts and components.

The spherical filling material also increases the notch toughness of thecured encapsulation compound.

The unavoidable remaining mechanical shrinkage stresses in the componentcan be absorbed by the filled casting resin by the filler being inspherical form in the epoxy resin, as a result of which the mechanicalcharacteristic values of the corresponding mixture are comparativelyconsiderably higher.

A further alternative method which can be used in its own right ortogether with the method mentioned above is the corresponding use ofhollow balls. In conjunction with the initially mentioned measure, thiswould result in a mixture of solid and hollow balls. The exclusive useor joint use of hollow balls makes it possible to produce an insulatorof low density, which allows the subsequent overall component to have alow total weight.

A further alternative which, however, can also be read as being optionalin conjunction with the above claims may consist in that at least oneswitching chamber is provided with a cast surround composed of a firstencapsulation compound, and is then encapsulated together withconnections into a block composed of at least one second encapsulationcompound such as silicone, soft epoxy or plastic.

Epoxy resin is used as the first encapsulation compound, and silicone,polyurethane, a polyurethane derivative or soft epoxy is used as thesecond encapsulation compound.

In this case, in a further advantageous refinement, it is now possibleto provide for the particles to be incorporated both into the first andinto the second encapsulation compound.

In one advantageous refinement, the balls or the hollow balls arecomposed of glass, or of ceramic, preferably of aluminum-nitrideceramic. A material which is suitable for use in electrical switchingdevices is thus chosen.

It is also advantageous for the filling level to be set to be between 50and 90%. This allows optimum results to be achieved with regard tomechanical requirements and crack-prevention measures.

In a further advantageous refinement, other fillers are mixed with theball and/or hollow ball mixture.

Commercially available cinder pastes or else primers can be applied tothe glass surface in order to achieve better wetting of the glass ballsor else hollow glass balls. In the future, a novel combination ofdifferent filling powders in the epoxy resin mixture will make itpossible to encapsulate the inserted parts (for example vacuum switchingchambers or other metallic or non-metallic inserted parts) withoutcushioning, or else directly with the epoxy resin mixture by means ofsuch a combination.

In order to achieve optimum results, external diameter mixtures of theballs or hollow balls are firstly used with a bandwidth of 65micrometers to 120 micrometers. Furthermore, optimum results are alsoachieved with external diameters from 40 micrometers to 85 micrometers.

In a further advantageous refinement, the particles have a mean densityof 0.2 g/cm³.

In one advantageous refinement, the particles have a mean density of0.37 g/cm³.

Further refinements in which good mechanical and electricalcharacteristics can be achieved as a result are as follows:

-   -   hollow balls with a diameter of up to 200 micrometers.    -   hollow balls with an effective density between 0.1 and 0.6        g/cm³.    -   solid balls with a density between 2.0 and 7.0 g/cm³.

The density of the hollow balls as mentioned above means the effectivedensity, that is to say the weight per unit volume with regard to thecavity.

The features of the device according to the invention are designed in acorresponding manner.

A further aspect is the improvement of the thermal conductivity whenheat is produced in the switchgear assemblies. This heat must be passedfrom the inside to the outside, that is to say it must be dissipated.

Fillers or additives with a high specific thermal conductivity arechosen for this reason. Overall, a material such as this or a componentmanufactured from it is considerably more suitable than epoxy resin orsome other plastic on its own. At the same time, the particle fillingaccording to the invention reduces the crack sensitivity and results ina good isolation effect.

First of all, irrespective of the additives, it is possible to provideenveloping overall encapsulation in silicone or in a soft epoxy casing,which surrounds the switching chambers on the one hand and theconnections on the other hand.

The invention will be described in more detail in the following text andis illustrated in the drawing, in which:

FIG. 1 shows a pole part with a vacuum switching chamber,

FIG. 2 shows a component according to FIG. 1, in a three-phase version.

FIG. 3 shows an embodiment with block encapsulation in, for example,silicone.

By way of example, FIG. 1 shows a pole part of a switch gear assembly.In this case, a vacuum switching chamber 1 is encapsulated by a firstencapsulation compound 10 composed of epoxy resin.

As already stated, the chosen encapsulation compound is preferably epoxyresin and, in the wording of the patent claims, is referred to as thefirst encapsulation compound. According to the invention, this can nowalso be provided with balls or particles of the stated size. The effectof reduction in the risk of crack formation according to the inventionis achieved at the same time as good thermal conductivity. In order toachieve optimum thermal conductivity, the particles or balls arepreferably composed of aluminum nitride. Aluminum oxides are alsosuitable, but the thermal conductivity of AlN is greater than that ofAl₂O₃.

FIG. 2 shows a three-phase switching arrangement for a three-phasesupply. In this case, epoxy, silicone or polyurethane is used as thefinal enveloping material, that is to say as the second encapsulationcompound 20, into which the pole parts together with theconnections/busbars 2, which have been encapsulated with the firstencapsulation compound, are inserted and are enveloped/encapsulated bythe second encapsulation compound 20. Injection-molding methods can alsobe used in this case. Epoxy, silicone or polyurethane is used. This isthen provided with the filler, in the described manner.

The fillers, that is to say the balls, hollow balls and further fillers,are introduced into the stated material. A statistical distribution ofselected particles and ball sizes leads to a high packing density.

This now means that a combination of different balls, glass balls orhollow glass balls is used as an additive to the epoxy resin compound inorder to reduce internal stresses in epoxy resin components in thepresence of inserted parts (for example vacuum switching chambers orother parts) and to absorb unavoidable mechanical stresses. The fillinglevel governs the mechanical and thermal characteristics. This ispreferably 50-90%. The density of the epoxy resin mixture isconsiderably reduced by the use of hollow glass balls. The mechanicalstrength of the component and of the encapsulation compound areincreased by the addition of balls, glass balls or hollow glass balls tothe silicone, polyurethane or “soft” casting resin.

The invention also provides for the additional mixing of further fillingcomponents with the balls in a corresponding mixture ratio, for examplequartz powder, synthetic silica flour or Wollastonite.

A different thermosetting plastic molding (for example polyurethane) canalso be used instead of epoxy resin.

In this case, the hollow glass balls can be kept in contact with oneanother in the encapsulation compound such that the epoxy resin,silicone or polyurethane in consequence fills only the gaps between thehollow glass balls, without any bubbles. The thermal coefficient ofexpansion decreases towards that of glass.

If a system composed of silicone or polyurethane, or a “soft” castingresin is chosen, then it is possible by the addition of balls, glassballs or hollow glass balls to achieve a considerable increase in themechanical strength of the component. This will make it possible in thefuture to provide material mixtures such as these as constructionmaterials for the encapsulation of mechanically stressed isolators(component) with the required attachment points. Furthermore, the use ofhollow glass balls will make it possible to produce extremely“lightweight” components with high mechanical and dielectric strength.

One particular problem that arises in the production of solid insulationfor an isolator block (for example for encapsulation of all thecomponents of a switchgear assembly) is that the heat that is producedmust be passed to the surrounding area outwards through the isolator inorder to ensure that the temperature of the encapsulated components doesnot exceed a maximum permissible value. At the moment, this is achievedby the measure of the isolator wall thickness being small but adequateand/or by the fitting of a heat exchanger composed of metal (passingthrough the isolator at one point as far as the metal parts, or into thevicinity of a metal part).

A switchgear assembly with solid insulation contains not only theswitching elements, for example the pole parts, but also a number ofconnections and electrical junctions which must themselves likewise havesolid insulation and must be sealed dielectrically at the junctionpoints by means of appropriate isolation elements.

In contrast, all of the necessary components such as the vacuumswitching chamber as an active switching element, a three-positionswitch—possibly in the form of a further vacuum chamber—, the powersupply busbars, transformers and further components are introduced intoan optimized volume. All of the equipment is then encapsulated in a moldto form a “block” or a unit, preferably using silicone rubber. In orderto allow the heat to flow from the area inside the block that is createdto the outside, a ceramic filler can be incorporated in the silicone.The filler can be incorporated in the silicone compound beforehand.Another option is impregnation of the filler, for example with silicone,in the evacuated mold.

The use of silicone as the encapsulation compound makes it possible toenclose an entire technical device with one isolator, without any crackformation.

The connections to a possible three-phase “block” are preferablyprovided by means of cables connected to commercially available plugconnections of the respective plug sizes. The sockets are permanentlyconnected and encapsulated in or on the “block”. Casting resincomponents (pole parts, and the like) can also extend into the siliconecompound in order to increase the mechanical strength in the area of theswitching elements, as shown in the two sketches. An appropriate drive,for example, can be mounted on these from the outside. The other parts(power supply busbars, transformers, etc.) are mounted between thecomponents. After application of appropriate adhesion promoters, anelectrically “sealed” block can be produced with all the necessarycomponents.

The heat flow which occurs inside the block can be passed to the blocksurface in particular by means of a filler composed of the material AlN(up to 220 W/mK). If a high filling proportion of this ceramic materialis introduced into the silicone material, the thermal conductivity ofthe encapsulation compound can be increased considerably, and thedielectric performance can be kept at today's level, or can beincreased. The heat flow can be dissipated to the exterior by means ofappropriate enlargement of the surface area (ribs associated withconvection in the surrounding air) and/or by means of cooling elementsat appropriate dielectrically non-critical points.

If, for example, the filler is introduced directly into a mold whichsurrounds the components, comparatively “large” particle diameters canbe chosen for the ceramic components. This means particle sizes, forexample, in the range from 1 to 10 mm, preferably with a spherical shapein order to increase the notch toughness on the complete block. This isdifferent to the situation of encapsulation with a prefabricatedencapsulation compound. In this situation, appropriately finer particlesmust be chosen in order to ensure sufficiently low viscosity for thesubsequent processes.

For simplicity, individual blocks can also be produced instead of anentire block in which all of the components are located. The use ofindividual blocks allows a maintenance-friendly and low-cost solutionfor repair purposes.

FIG. 3 shows the transparent illustration of the incorporation of allthe described elements in enveloping block encapsulation of the secondencapsulation compound 20, for example with silicone or soft epoxy. Inthis case, both the switching chambers 1 and the connections or busbars2 are also encapsulated. The physical arrangement of the pole parts 1leads to mechanical stiffening of the block in the encapsulated blockarrangement, although it is composed of the soft second encapsulationcompound.

In a three-phase version, as shown in FIG. 2, the blocks are separatedfrom one another by means of intermediate plates 3, for heatdissipation.

1. A method for production of moldings for switching devices forlow-voltage, medium-voltage and high-voltage, characterized in that amixture of balls with a predetermined distribution of diameters of sizeDx is introduced into the encapsulation compound thus creating directencapsulation of components.
 2. A method for production of moldings forswitching devices for low-voltage, medium-voltage and high-voltage, inparticular as claimed in claim 1, characterized in that a mixture ofhollow balls with a predetermined distribution of external diameters ofsize Dx is introduced into the encapsulation compound.
 3. A method forproduction of switching devices for low-voltage, medium-voltage andhigh-voltage, in particular as claimed in claim 1 and/or 2,characterized in that at least one switching chamber is provided with acast surround composed of a first encapsulation compound, and is thenencapsulated together with connections into a block composed of at leastone second encapsulation compound such as silicone, soft epoxy orplastics.
 4. The method as claimed in claim 1, 2 or 3, characterized inthat epoxy resin is used as the first encapsulation compound, andsilicone, polyurethane or a polyurethane derivative is used as thesecond encapsulation compound.
 5. The method as claimed in claim 4,characterized in that the particles are introduced into the first and/orinto the second encapsulation compound.
 6. The method as claimed in oneof the preceding claims, characterized in that the balls or the hollowballs are composed of glass.
 7. The method as claimed in claim 1, 2 or3, characterized in that the balls or the hollow balls are composed ofceramic, preferably of aluminum nitride.
 8. The method as claimed in oneof the preceding claims, characterized in that the filling level is setto be between 50 and 90%.
 9. The method as claimed in one of thepreceding claims, characterized in that other fillers in the form ofsmall particles are mixed with the ball and/or hollow ball mixture. 10.The method as claimed in one of the preceding claims, characterized inthat the other fillers are quartz powder or synthetic silica flour. 11.The method as claimed in one of the preceding claims, characterized inthat the external diameters of the balls or hollow balls or particleshave a bandwidth of 0.01 mm to 10 mm.
 12. The method as claimed in oneof the preceding claims, characterized in that the balls, hollow ballsor particles have a mean density of 0.2 g/cm³.
 13. The method as claimedin one of the preceding claims, characterized in that the balls, hollowballs or particles have a mean density of 0.37 g/cm³.
 14. The method asclaimed in one of the preceding claims, characterized in that the hollowballs have a diameter of up to 200 micrometers.
 15. The method asclaimed in one of the preceding claims, characterized in that the hollowballs have an effective density between 0.1 and 0.6 g/cm³.
 16. Themethod as claimed in one of the preceding claims, characterized in thatthe solid balls have a density between 2.0 and 7.0 g/cm³.
 17. Aswitching device for low-voltage, medium-voltage and high-voltage,having encapsulated moldings, characterized in that a mixture of ballsand/or hollow balls and/or particles with a predetermined distributionof diameters of size Dx is introduced into the first encapsulationcompound thus creating direct encapsulation of moldings, and themoldings of a switching device are composed of electrically insulatingmaterials.
 18. A switching device for low-voltage, medium-voltage andhigh-voltage, having encapsulated moldings, characterized in that thesecond encapsulation compound in which the moldings with cast surroundsare inserted and/or are once again encapsulated in this way is composedof electrically insulating materials, such as silicone, epoxy resin orpolyurethane.
 19. The switching device as claimed in claim 17 or 18,characterized in that at least one switching chamber is provided with acast surround composed of a first encapsulation compound, and is thenencapsulated together with connections into a block composed of at leastone second encapsulation compound such as silicone, soft epoxy orplastics.
 20. The switching device as claimed in one of claims 17 to 19,characterized in that epoxy resin is used as the first encapsulationcompound, and silicone, polyurethane or a polyurethane derivative isused as the second encapsulation compound.
 21. The switching device asclaimed in claim 20, characterized in that said particles or balls areintroduced into the first and/or into the second encapsulation compound.22. The switching device as claimed in claim 21, characterized in thatthe balls or hollow balls are composed of glass or ceramic.
 23. Theswitching device as claimed in one of claims 17 to 22, characterized inthat the balls or hollow balls are composed of aluminum-nitride ceramic.24. The switching device as claimed in one of the preceding claims 17 to22, characterized in that the moldings or components of a switchingdevice for each phase of a three-phase supply are each encapsulated toform a sealed block.
 25. The switching device as claimed in claim 24,characterized in that the respective block is provided withheat-dissipating connection elements (2).